Model 18i / 14i / 12i Cryogenic
Temperature Monitors

Specifications

Model 18i / 14i / 12i General Specifications

Input Channels

The Model 18i has eight input channels where the Model 14i has four and the Model 12i has two. In every other way, the monitors are identical. Each input may be independently configured for any of the supported
sensors. There is no use of scanning or multiplexing.

Data logging is performed to an internal 2GB circular buffer and is
time-stamped with a real-time clock. Buffer memory is non-volatile
and will retain valid data without AC power. All eight input channel
temperatures are recorded.

Silicon and GaAlAs Diode sensors
from Cryo-con or any other manufacturer are directly supported over
their full 1.4 to 500K range using built-in calibration curves and
sensor data. Plus, non-volatile Flash memory is available for several
custom or calibrated sensors.

Platinum RTD sensors can
use built-in DIN 43760 (IEC 750) standard setups for 100Ω or
1,000Ω devices. The DIN standard is used for temperatures
from 70K to 1020K and extends it down to 30K for cryogenic use. A unique
feature of Model 18i / 14i / 12i is the use of an AC ratiometric resistance bridge to cancel low frequency noise.

The Model 18i / 14i / 12i provides robust support for the Negative
Temperature Coefficient (NTC) sensors commonly used by cryogenic applications.
They include Ruthenium-oxide, Cernox™, Carbon-Glass,
Germanium, Carbon-Ceramic and several others. Since they have
a negative temperature coefficient, the constant-voltage measurement
method will reduce, rather than increase, power dissipation in the
sensor as temperature decreases. By maintaining the lowest possible
power level, sensor self-heating is minimized and useful temperature
range is greatly increased.

An additional advantage to constant-voltage biasing
is that NTC resistors lose sensitivity in the upper part of their range.
By auto-ranging excitation current to maintain a constant voltage,
sensitivity and noise immunity in that range is greatly improved.

Sensor excitation used in conjunction with the constant-voltage
feature is a 1.625Hz bipolar square wave. This effectively cancels thermal
EMF induced offset errors that sometimes occur in cryogenic systems.

Thermocouple sensors are supported by using an optional external module that provides amplification, cold-junction compensation and connection to copper. The module is powered by the monitor and connects directly to any of the sensor inputs.